During the machining of a mold, etc., a thin end mill with a tool diameter of approximately from 8 mm to 20 mm has conventionally been used. In order to round a corner of a casting, a curved surface (referred to as a “corner R” or the like) needs to be formed between a bottom surface of the mold and a wall surface thereof.
Patent Document 1 discloses an insert 1 which comprises corner parts 11 each having a high curvature radius in order for such corner R to be formed.